Japan’s Underwater Aircraft Carriers – part one

Lieutenant Commander Stephen L. Johnson had a problem on his hands; a very large problem. His Balao-class submarine, the Segundo, had just picked up a large radar contact on the surface about 100 miles off Honshu, one of Japan’s home islands, heading south toward Tokyo. World War II in the Pacific had just ended, and the ensuing cease fire was in its 14th day. The official peace documents would not be signed for several more days.

As Johnson closed on the other vessel, he realized it was a gigantic submarine, so large in fact that it first looked like a surface ship in the darkness. The Americans had nothing that size, so he realized that it had to be a Japanese submarine.

This was the first command for the lanky 29-year-old commander. He and his crew faced the largest and perhaps the most advanced submarine in the world. The Japanese I-401 was longer than a football field and had a surface displacement of 5,233 tons, more than three times the Segundo’s displacement. More troubling though was the sub’s bristling weaponry that included a 5.5-inch gun on her aft deck, three triple-barreled 25mm antiaircraft guns, a single 25mm gun mounted on the bridge, and eight large torpedo tubes in her bow.

During a brief ceremony aboard one of the aircraft carrier submarines, the Japanese naval ensign is lowered and replaced by the Stars and Stripes as the vessel is turned over to the control of the U.S. Navy after Japan’s surrender

The large sub displayed the mandatory black surrender flag, but when the Segundo edged forward, the Japanese vessel moved rapidly into the night. The movement and the continuing display of the Rising Sun flag caused concern. Johnson’s vessel pursued the craft that eventually slowed down as dawn approached. He brought his bow torpedo tubes to bear on the craft as the two vessels settled into a Mexican standoff.

Johnson and his crew had received permission by now to sink the reluctant Japanese vessel if necessary, but he realized he had a career-boosting and perhaps a technologically promising prize in his sights. Much depended on this untried American submarine captain and his wily opponent in the seas off Japan.

Little did Johnson know that the Japanese submarine was a part of the I-400 squadron, basically underwater aircraft carriers, and that the I-401 carried Commander Tatsunosuke Ariizumi, developer of the top-secret subs initially designed to strike the U.S. homeland in a series of surprise attacks. Ariizumi was considered the “father of the I-400 series” and a loyal follower of the emperor with years of experience in the Japanese Navy, so surrender was a disgrace he could not endure

Johnson also had to contend with Lt. Cmdr. Nobukiyo Nambu, skipper of the I-401, who traced his combat experience back to Pearl Harbor. He now commanded the world’s largest submarine designed to carry three state-of-the-art attack planes in a specially built hanger located atop the vessel. These secret Aichi M6A1 planes were initially designed for “a second Pearl Harbor” or another surprise attack, possibly even against New York City or Washington, D.C. The I-400 series submarines were themselves full of technological surprises. They were capable of traveling around the world one and a half times without refueling, had a top surface speed of 19 knots (or nearly 22 miles per hour), and could remain on patrol for four months, twice as long as the Segundo.

Neither Nambu nor Commander Ariizumi readily accepted the emperor’s surrender statement when it was broadcast on August 15. The subsequent communiqués from Tokyo were exceptionally confusing, especially Order 114, which confirmed that peace had been declared – but that all submarines were to “execute predetermined missions and attack the enemy if discovered.”

It was Admiral Isoroku Yamamoto, commander of Japan’s Combined Fleet and developer of the Pearl Harbor attack, who called for the construction of the I-400 series some three weeks after Pearl Harbor. Once Japan was committed to war, he believed that submarine aircraft carriers dropping bombs “like rain” over major U.S. cities would surely cause the American people to “lose their will to fight.” A second surprise attack with even more to come would prove psychologically devastating to the Americans.

Yamamoto called for the construction of 18 of the massive submarines carrying a total of 36 attack planes. The name of the special submarine class was abbreviated to Sen-toku.

The attack planes had to be designed from scratch. The need for speed, range and a decent sized bomb payload required tradeoffs. The wings had to be foldable to fit inside the tube, or hangar, atop the submarine. The design work, testing, and building of the plane was outsourced to the Aichi Aircraft Company.

The I-400 program did have its detractors in the heavily bureaucratic Imperial Japanese Navy. After the defeat at Midway in early June 1942, Japan became more focused on defending the homeland and far less on possible attacks on the U.S. mainland using the large submarines. The death of Yamamoto in mid-April 1943, played further into the hands of conservative Japanese commanders. Cutbacks were ordered in the number of submarines to be built. .

The first test flight of the Aichi attack plane occurred on November 8, 1943. The plane, called Seiran or “storm from a clear sky,” reportedly handled fairly well as the world’s first sub-borne attack bomber. The Japanese began compiling limited available information on the heavily fortified Panama Canal. Their analysis showed that destroying the gate opening onto Gatun Lake would create a massive outpouring of water, destroying the other gates in its path while rushing toward the Caribbean Sea.

After weeks of planning, the Japanese came up with a strategy to attack the Gatun locks at dawn when the gates were closed and presumably the defenses were lax. The planners had nearly a full year to formulate the attack for early 1945. But there were problems ahead because none of the submarines were complete and the planes were not yet in the production stage.

I-400 Class submarine

The Japanese labored on, and by the end of 1944 the I-400 and the smaller I-13 were completed and turned over to the Navy. In early January 1945, the I-401 was commissioned and the I-14, the last of the underwater aircraft carriers, was put into service by mid-March 1945.

As an important aside, it should be noted that while preparations for the attack on the Panama Canal went forward, Vice Admiral Jisaburo Ozawa, vice-chief of the Naval General Staff, floated another idea for the use of the Sen-toku submarines. He suggested arming the Seiran planes with biological weapons to be unleashed against a populated area on the West Coast of the United States.

Dr. Shiro Ishii, Japan’s top virus expert and head of the Army’s notorious 731 unit in Manchuria, was consulted. He recommended that the planes drop plague-inflected fleas, something he had tested with success in China, on the United States with San Francisco, Los Angeles, or San Diego suggested as targets. The plan was discarded in late March by the head of the Army’s general staff who called it “unpardonable on humanitarian grounds.”

In effect, the Japanese Army, which had led the development of biological weapons and had tested them on Chinese and American captives, nixed the idea of using the weapons late in the war on American civilians, perhaps in the belief that the war was already lost.

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About GP Cox

Everett Smith served with the Headquarters Company, 187th Regiment, 11th A/B Division during WWII. This site is in tribute to my father, "Smitty."
GPCox is a member of the 11th Airborne Association. Member # 4511 and extremely proud of that fact!